1. Field of the Invention
The present invention relates to a synchronous motor driving system, and more particularly, it relates to a control method for achieving a highly precise and high-performance synchronous motor driving system without using any sensors for detecting a rotational speed of a synchronous motor and a position of a magnetic pole.
2. Description of the Related Art
Many developments have been made for a method for controlling a synchronous motor without detecting the rotational speed of the synchronous motor and the position of a magnetic pole. Such control methods are usually classified into two types.
A first type is a control method based on speed/position sensor vector control of the synchronous motor. Instead of using the speed/position sensor, a magnetic pole position estimating instrument, and a speed estimating instrument are used. For example, a method is known, which is described in a document 1: xe2x80x9cNo. SPC-00-67: A New Position Sensorless Control of IPM Synchronous Motor using Direct Position Error Estimationxe2x80x9d, by inventors Kiyoshi Sakamoto, Yoshitaka Iwazi, and Tsunehiro Endo, in xe2x80x9cIEEJ Semiconductor Power Conversion/Industrial Electric Power Application Joint Research Materialxe2x80x9d (November, 2000). This method is known as a vector control sensorless system.
A second type is a control method called a V/F control system, which controls the synchronous motor by an open loop.
In the case of the vector control sensorless system, except for non-presence of a position/speed sensor, a configuration itself of the control system is similar to that of a vector control system equipped with a sensor. Accordingly, a high-performance synchronous motor driving system can be achieved.
FIG. 15 shows a relation between a d-q coordinate axis and an assumed axis dc-qc by a vector with a magnetic pole axis of a synchronous motor set as a reference axis. For vector control, as shown in FIG. 15, a magnetic pole axis of the synchronous motor is set as a d axis, an axis orthogonal to the same as a q axis. Then, by properly controlling a voltage and a current applied to the synchronous motor on each axis, high-performance making utmost use of synchronous motor performance is achieved. According to this vector control sensorless system, torque can be made linear, and efficiency can be maximized.
In the case of the vector control sensorless system, a dc-qc axis is set by assuming a d-q axis on control, deviation (axial error) xcex94xcex8 from a real d-q axis is estimated, and a dc-qc axial phase is adjusted to reduce the deviation to zero. Thus, in the case of the vector control sensorless system, a method of estimating an axial error xcex94xcex8 is a most important factor for deciding control performance.
In well-known examples, several estimation methods of axial errors xcex94xcex8 have been presented according to the rotational speed zones of the synchronous motor. In practice, all the speed zones are covered by using these control methods in association.
On the other hand, in the case of the V/F control system, no speed or current automatic adjustment units are provided, and a voltage to be applied to the synchronous motor is decided. As its conventional example, a control method is described in JP-A-2000-236694. In the case of the V/F control, different form the case of the vector sensorless system, a magnetic pole axis is not estimated. Thus, a configuration of a control system is greatly simplified. However, if a load is suddenly changed during driving, transient vibration may occur. In order to suppress such transient vibration, JP-A-2000-236694 presents a control system for correcting a speed based on a current detected value.
In the case of the vector control sensorless system, a sensorless system must be switched to another according to a driving speed of the synchronous motor. The method described in the document 1 estimates an axial error xcex94xcex8 based on a speed electromotive voltage of the synchronous motor in principle, which can be achieved only in a middle/high speed zone. A similar problem is inherent in the method described in JP-A-8-308286.
On the other hand, as a sensorless system of a low speed, many methods have been presented, which uses a inductance difference (saliency) of the synchronous motor. For example, as described in JP-A-7-245981, there is a method for superposes a higher harmonic wave on a voltage command, and calculates an axial error based on a higher harmonic current component thus generated.
In this method, however, since it is necessary to superpose the higher harmonic wave, a pulsation component is generated in a synchronous motor current, causing a considerable reduction in efficiency of the synchronous motor. In addition, because of the superposed wave, electromagnetic noise is increased. Especially, to detect an axial error with high sensitivity, the amount of superposed waves to be injected must be increased, and thus it is difficult to solve the above-described problems, and achieve high control performance at the same time.
In addition, since the saliency of the synchronous motor is used, the system cannot be applied to a synchronous motor of a non-saliency type. Further, when the low-speed system, and the middle/high speed system are used in association, the systems must be switched according to a speed, and thus shocks occur following the switching.
On the other hand, in the case of the V/F control, the synchronous motor can be driven from a low to high speed zone by a configuration of a single control system.
However, in the V/F control, since the d-q axis in the synchronous motor is not basically coincident with the dc-qc axis on control, it is difficult to achieve high-performance control. For example, it is difficult to achieve high-speed response to a change in a rotational speed command, linear control of torque, maximum efficiency control and the like. Accordingly, there is a possibility that external disturbances such as fluctuation in load torque may cause inconveniences such as vibration or excessive current.
An object of the present invention is to provide a synchronous motor driving system equipped with means for stabilizing a control system without superposing any higher harmonic waves, controlling a low-speed zone to a high-speed zone by a continuous method, and achieving a vector control sensorless system.
The foregoing object of the present invention can be achieved by providing means for calculating an error xcex94xcex8 as deviation between a magnetic pole axis in a synchronous motor and a magnetic pole axis on control, as a function of a driving current command, a detected current value, an inductance constant, and generated power constant of the synchronous motor, calculating an axial error by applying the axial error calculating means to all speed zones of the synchronous motor except zero, and correcting the magnetic pole axis on the control based on the axial error.
That is, in order to achieve object, in accordance with the present invention, there is provided a synchronous motor driving system which comprises a synchronous motor, an inverter for driving the synchronous motor, a rotational speed command generator for supplying a rotational speed command to the synchronous motor, and a control unit for calculating a voltage applied to the synchronous motor, said synchronous motor driving system comprising axial error calculation means for estimating an axial error xcex94xcex8 between a d-q axis and a dc-qc axis by using Ld, Lq, Ke, Id*, Iq*, Idc and Iqc in a range of all rotational speeds except zero of the rotational speed command of the synchronous motor wherein Ld is an inductance on a magnetic pole axis d, Lq is an inductance on a q axis orthogonal to the magnetic pole axis d, Ke is a generated power constant of the motor, Id* is a current command of the d axis, Iq* is a current command on the q axis, Idc is a detected current value on an assumed dc axis on control, and Iqc is a detected current value on an assumed qc axis orthogonal to the assumed dc axis; and means for adjusting the dc-qc axis to the d-q axis based on the calculated value of the axial error xcex94xcex8.
Furthermore, in order to achieve the above object, in accordance with the present invention, there is provided a synchronous motor driving system which comprises a synchronous motor, an inverter for driving the synchronous motor, a rotational speed command generator for supplying a rotational speed command to the synchronous motor, and a control unit for calculating a voltage applied to the synchronous motor, said synchronous motor driving system comprising axial error calculation means for estimating an axial error xcex94xcex8 as a function of an inductance L and a generated power constant Ke among the resistance R, the inductance L and a generated power constant Ke as synchronous motor constants of the synchronous motor in a range of all rotational speeds except zero of a rotational speed command of the synchronous motor; and means for adjusting the dc-qc axis to the d-q axis based on the calculated value of the axial error xcex94xcex8, wherein Ld is an inductance on a magnetic pole axis d, Lq is an inductance on a q axis orthogonal to the magnetic pole axis d, Ke is a generated power constant of the motor, Id* is a current command of the d axis, Iq* is a current command on the q axis Iq*, Idc is a detected current value on an assumed dc axis on control, and Iqc is a detected current value on an assumed qc axis orthogonal to the assumed dc axis.
The axial error calculation means can be adapted to calculate an axial error xcex94xcex8 by using current commands Id* and Iq* on the d-q axis, and detected current values Idc and Iqc on the dc-qc axis, wherein Ld is an inductance on a magnetic pole axis d, Lq is an inductance on a q axis orthogonal to the magnetic pole axis d, Ke is a generated power constant of the motor, Id* is a current command of the d axis, Iq* is a current command on the q axis, Idc is a detected current value on an assumed dc axis on control, and Iqc is a detected current value on an assumed qc axis orthogonal to the assumed dc axis.
In this case, instead of the detected current value Idc on the dc axis, a current command Id* can be used.
In addition, the synchronous motor driving system may be provided with means for detecting a DC current on a power source side of the inverter, and synchronous motor current estimating means for estimating an AC current of the synchronous motor based on the detected DC current and a driving pulse signal for driving the inverter, and the axial error xcex94xcex8 may be calculated using the estimated current as a detected current value.
The synchronous motor driving system can be provided with means for detecting a DC current on a power source side of the inverter, and Iqc estimating means for estimating a current value on the qc axis of the synchronous motor based on the detected DC current, and a detected value or a set value of a DC voltage of the inverter, and the axial error xcex94xcex8 can be calculated using the estimated current as a detected current value.
In any of the above-described synchronous motor driving systems, in the calculation of the axial error xcex94xcex8, a correction term may be provided to make correction in accordance with a rotational speed command of the synchronous motor, and the correction term may be set as a function of weight which increases as the rotational speed command approaches zero.
The current command Iq* on the q axis can be made based on the detected current value or the estimated value on the qc axis.
The synchronous motor driving system can be provided with means for estimating a speed deviation between the rotational speed command and a real rotational speed based on the calculated value of the axial error xcex94xcex8, and the q axis current command Iq* of the synchronous motor can be made based on the estimated value of the speed deviation.
The synchronous motor driving system may be provided with means for estimating a rotational speed of the synchronous motor based on the calculated value of the axial error xcex94xcex8, and the q axis current command Iq* of the synchronous motor may be made based on a deviation between the estimated value and the rotational speed command.
In the above-described synchronous motor driving systems of the three types, the current command Id* of the d axis can be made based on the current command Iq* of the q axis.
In the present invention, the synchronous motor may be a salient type or a non-salient type.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.